Interferometers of the kind mentioned above, i.e. fibre-optic Mach - Zehnder interferometers, are described in Technisches Messen 51. Jahrgang 1984 Heft 6, R.Kist: "Messwerterfassung mit faseroptischen Sensoren". In these instruments light is sent from a common light source through two optical fibres, and the phase angles for the light waves in the two fibres are compared after having passed through the fibres. If one optical fibre is subjected to a change in length, for example, the phase difference between the light waves changes, and this change is a measure of the change in length. In a known apparatus, the ends of the two fibres are fused together and connected to a detector for the light intensity, in order to compare the phase angles of the light waves in the fibres. The intensity detected here is dependent on the phase displacement. The apparatus is simple but gives poor measurement accuracy in the interferometer since the detected intensity is also dependent on changes in the light source intensity and on the light intensities in the two fibres. In another known apparatus, the light from the two fibres illuminates at a sloping angle of incidence each side of a semitransparent mirror. The intensities of the two light waves departing from the mirror are detected individually and compared. An interferometer with this type of apparatus for enabling the comparison of the phase angles gives relatively good measuring accuracy, which is limited, however, by not being able to use polarized light. The apparatus is also delicate and voluminous, which limits the field of use of the interferometer.